Many electronic devices using a battery power supply are being developed such as portable computers, mobile telephones, electronic cameras, etc. Such appliances need to be light weight and use little power. It is known however, that long term usage of rechargeable batteries gives rise to various problems.
Firstly, there is no way for the user to tell how much capacity is left in the rechargeable battery during use. He can only speculate on the time when recharging is required and for how long recharging is needed. If the user fails to judge correctly when the batteries need to be recharged the loss of capacity may only become apparent in the middle of operating the appliance. This may be especially inconvenient when the battery is a heavy weight or large volume device such that a spare battery is not carried when operating the appliance outdoors.
As a consequence, an important picture or news item might be missed in the case of batteries failing in a portable camera, or a portable computer might fail during running, or the conversation on a mobile telephone might be interrupted, etc.
Secondly, during charging of a battery, there is no way of knowing whether the capacity of the battery has been saturated, particularly since the time taken for saturated charging may vary depending on the structure of the charger and many different chargers are available. As a result, the user may not know when the battery is actually fully re-charged.
Thirdly, in the case of fast recharging batteries that are recharged within a short time, speculation or guess work on the part of the user might easily lead to over-charging and even to damaging of the batteries. It is possible, for example, for there to be 50% of the capacity left at the start of recharging, so the risks of either inadequate or over-saturated re-charging still exist.